Nematode diseases

Introduction

Nematodes belong to the Phylum Nematoda. There are several hundred species that can cause diseases in crops and they can also be important in the infection process by soil-borne fungi and bacteria by providing entry sites where the nematodes wound root tissue. Following Meloidogyne spp. infection, nutrient sinks are formed within the roots encouraging the entry of other pathogens and host defences against vascular infection are also reduced.

Nematode problems in Vietnam range from the foliar nematode, Aphelenchoides oryzae Yokoo, causing white tip of rice leaves, to the stem nematode, Ditylenchus anguatus (Butler) Filipjev, infecting floating rice in the Mekong Delta, h owever most plant pathogenic nematodes however cause diseases in the root systems of various crops, as they survive and are disseminated in the soil.

There are two major species of root knot nematodes in Vietnamese crops, Meloidogyne spp. and Heterodera spp., where Meloidogyne spp. are the most important. There are four major species of nematodes that cause root rots, Pratylenchus spp., Tylenchulus spp., Paratylenchus spp. and Radophilus spp.. Radophilus similus (Cobb) Thorne is a major banana pathogen not yet introduced in Vietnam. It is the focus of banana quarantine practices in Vietnam.

Disease Symptoms

This disease can be diagnosed from the symptoms below the ground. Above ground symptoms will generally consist of stunting and wilting when there is insufficient moisture. Roots infected with Meloidogyne will develop knots and infection by Heterodera spp. infection leads to cysts. The growths contain white mature female nematodes of many shapes and sizes, depending on the population density, root size and environmental conditions. Those plants infected with the lesion nematode Pratylenchus spp., the citrus nematode Tylenchulus spp., the pin nematode Paratylenchus spp. or the burrowing nematode Radopholis spp. will develop necrotic lesions, where infected.

Description of the Pathogen

The male nematodes can be up to 4 mm long and 15-35 µm wide while the females of Meloidogyne spp. and Heterodera spp. become lemon-shaped upon maturity. They cannot be seen with the naked eye, but are easily observed under the microscope, particularly when stained. Plant parasitic nematodes obtain their food through spears or stylets which enable iinfection. The life cycle begins from an egg, which hatches into a juvenile and after four moults differentiates into an adult male or female. The female can produce eggs after mating or parthenogenetically. The life cycle can take 2-4 weeks, longer in cooler conditions.

Distribution within Vietnam

These diseases is present in all parts of Vietnam.

Host Range

There are many hosts of these pathogens, and species are not host specific, for example, the exotic burrowing nematode Radophilus similis infects banana, durian, black pepper and many other crops. R. citrophilus is an important pathogen of citrus. Many broad-leved crops are hosts of the root knot nematode, Meloidogyne spp., with cereals also hosts in Vietnam. Heterodera spp. are commonly known as the cyst nematode of tobacco, soybean, sugar beets and cereals. Pratylenchus spp. cause lesions on most crop plants and trees.

Tylenchulus spp. infect citrus, grapes and olives. Species within the genus Radophilus spp. infect banana, citrus, coffee and sugarcane while Paratylenchus spp. has a wide host range.

Epidemiology

Nematodes survive in the soil as juveniles, and prefer temperatures above 15°C for infection. Nematodes survive and disperse in the soil, where temperature, moisture and aeration affect their survival, movement and infection. They are most abundant in the top 15-30 cm of soil. In cultivated soil, the distribution of nematodes is irregular and concentrated around the base of susceptible plants.

There are two main groups of soil-borne plant parasitic nematodes, the endoparasitic nematodes such as those that cause root knots and cysts which enter the host and feed from within the root, and ectoparasites that cause general lesions and necrosis. They do not normally enter root tissue, but feed from the outside on the cells near the root surfaces. Meloidogyne spp. are endoparasites, which penetrate and develop to maturity within the stele of the root. Pratylenchus spp. feed in the cortex and are able to leave the root, move into the soil and reinfect new host tissue. Tylenchorhynchus spp. are ectoparasitic, living outside the root and using a feeding stylet to penetrate the epidermis.

Control

Nematodes can be controlled by including paddy rice in the rotation. Quarantine of infected soil should be implemented to prevent the spread of the nematodes into new areas.

Root lesion nematode infection of wheat